I have the opposite problem - in my viewport I can’t see it, but when I render my sequence it’s there. Also the extra passes don’t have the motion blur which leaves those useless unfortunately. If we could get a velocity pass that’d be one way to make it work! To be honest, I’d love to have that even if I/we can figure this issue out.
I found out motion blur will be applied to the final render if you play the game in editor once. Also, I don’t know if it actually makes a difference, but I’ve set my sequence to Auto Play for it to play once in PIE too. So just hit play, wait for it to start playing and stop.
Perhaps PIE creates some required buffers or something that the Movie Render feature requires.